U.S. Pat. No. 3,978,030 to Resnick describes certain polymers of perfluoro-2,2-dimethyl- 1,3 -dioxole (PDD), which has the following formula (1): ##STR1##
The above patent describes both homopolymers of PDD, which are not further characterized, and a crystalline copolymer with tetrafluoroethylene (TFE), which has a melting point of about 2650.degree. C.
Since Resnick's discovery of PDD homopolymer, it has been established that the material is amorphous and has a very high glass transition temperature, T.sub.g, of about 3350.degree. C. The homopolymer, however, is brittle and difficult to fabricate into final products because of poor melt flow and low solubility in available solvents.
U.S. Pat. No. 4,530,569 to Squire describes amorphous dipolymers of PDD with TFE as well as terpolymers of PDD with TFE and another comonomer. U.S. Pat. Nos. 4,935,477 and 4,754,009, both to Squire, describe dipolymers and terpolymers (collectively, copolymers) of PDD with comonomers, which include, i. a., certain perfluoroolefins and perfluoro(alkyl vinyl ethers). It is shown in those patents that, in general, the T.sub.g of those copolymers decreases with increasing comonomer content, although not necessarily in a linear fashion. For a dipolymer containing 10 mole % of TFE, T.sub.g is decreased to about 260.degree. C. Generally speaking, if the PDD content in those copolymers is less than about 12 mole %, those copolymers have some crystallinity, which normally is not desirable. While the amorphous copolymers are soluble at room temperature in perfluoro(2-butyl tetrahydrofuran) available from 3M Company as FLUORINERT.RTM. FC-75, their solubility is not high. For example, a dipolymer with 10 mole % of TFE has a solubility of less than 3 weight percent in this solvent. This, naturally, imposes some restrictions on the fabrication techniques which require application of copolymer from solution, for example, dip-coating or spray-coating.
U.S. Pat. No. 4,897,457 to Nakamura et al. describes amorphous homopolymers and copolymers containing either one of the following repeating cyclic structures (2) or (3) ##STR2## formed during the cyclic polymerization of perfluoro(butenyl vinyl ether) (PBVE), CF2.dbd.CF--OCF2CF2CF.dbd.CF2. The homopolymer has a T.sub.g of only about 108.degree. C. Those polymers are soluble in perfluorobenzene as well as in FLUORINERT.RTM. FC-75. Comonomers disclosed appeared to reduce the T.sub.g. Moreover, the patent states that copolymers with either fluoroolefins or vinyl ethers lose transparency, solvent-solubility, and mechanical strength if the repeating units (2) or (3) constitute less than 80 wt % of the polymer.
It appears, however, that copolymerization of PDD with PBVE, if successful, could lead to new materials having potentially useful properties.